This invention concerns a process for the preparation of anthraquinone.
More specifically, it concerns a modified process for producing anthraquinone by cyclization of ortho-benzoylbenzoic acid in the presence of sulfuric acid.
In addition to its application in the area of dyestuffs, anthraquinone is used in greater amounts and more widely in industry, particularly in the papermaking industry, to improve the alkaline digestion of wood, as described for example in the French Patent published under No. 2,373,637.
The delignification catalyst role which anthraquinone plays in this context is currently obtained when this product is employed in a highly purified form, which must therefore be achieved first.
Unfortunately, this sort of anthraquinone preparation represents an obvious economic burden in an industry such as the papermaking industry which produces an end-product of low market value.
The technical problem which the applicant attempted to solve was to obtain a process producing anthraquinone economically and in a form in which it could be used in an industry such as the papermaking industry to guarantee economic gain without loss of efficiency.
Cyclization of ortho-benzoylbenzoic acid, hereinafter referred to as OBB acid, by heating in the presence of a large quantity of concentrated sulfuric acid or oleum (cf., for example, I. Vogel, "Practical Organic Chemistry", 3rd edition, p. 740), isolating the resulting anthraquinone and purifying it, for example, by causing it to precipitate by diluting the reaction mixture with a large volume of water, isolating the precipitated anthraquinone by filtration and then washing it thoroughly with water to neutral pH to eliminate as many as possible of the residual impurities before drying, is a known process. Anthraquinone purified in this manner is generally prepared before use in the papermaking industry in the form of a dispersion in a solid inert product containing 50% anthraquinone by weight and produced by finely grinding the two constituents in the presence of a dispersing agent.
The methods of cyclizing OBB acid to produce purified anthraquinone such as is currently used in industry entail not only a high direct cost for this product, but also an additional consequential cost for decontamination of acid effluents before disposal. This initial high cost for anthraquinone is obviously inconsistent with its application in an industry such as the papermaking industry, which requires low-cost raw materials.
Attempts have been made to make the production of anthraquinone from OBB acid more economical. These have involved, for example, either reducing the quantity of sulfuric acid as described in Japanese Patent Application No. 49-7260/74 or the U.S. Pat. No. 2,842,562, or replacing the sulfuric acid with another catalyst such as clay as proposed in French Utility Patent No. 76.18056, Japanese Patent No. 49-6240/74 or the French Patent Application published under number 2,545,483.
In all of the above cases, since the processes are designed to produce purified anthraquinone, the advantages obtained in terms of the lowest possible materials cost are economically cancelled out by the higher cost engendered by the greater complexity of the equipment required.